Melt butter in a large stockpot over medium-high heat. Add onion, celery, black pepper, salt, and mushrooms; sauté 4 minutes or until mushrooms are tender. Add sherry; cook 1 minute.
Weigh or lightly spoon flour into dry measuring cups; level with a knife. Gradually add flour to pan; cook 3 minutes, stirring constantly (mixture will be thick) with a whisk. Gradually add broth, stirring constantly. Bring to a boil. Reduce heat; simmer 5 minutes, stirring frequently. Remove from heat.
Add 1 ¾ cups Parmesan cheese and cream cheese, stirring with a whisk until cream cheese melts. Add pasta and chicken; stir until blended. Divide pasta mixture between 2 (8-inch-square) glass or ceramic baking dishes coated with cooking spray.
Place bread in food processor; pulse 10 times or until coarse crumbs form. Combine breadcrumbs and ½ cup Parmesan cheese; sprinkle evenly over pasta.
Bake at 350° for 30 minutes or until lightly browned and bubbly. Remove casserole from oven; let stand 15 minutes.
To freeze unbaked casserole: Prepare through Step Cool completely in refrigerator. Cover with plastic wrap, pressing to remove as much air as possible. Wrap with heavy-duty foil. Store in freezer for up to 2 months.
To prepare frozen unbaked casserole: Thaw casserole completely in refrigerator (about 24 hours). Preheat oven to 350°. Remove foil; reserve foil. Remove plastic wrap; discard wrap. Cover casserole with reserved foil; bake at 350° for 30 minutes. Uncover and bake an additional 1 hour or until golden and bubbly. Let stand 15 minutes.